Substitution of skeletal troponin C with cardiac troponin C in rabbit myofibrils increased relative Sr2+ sensitivity 10-fold and halved the maximal slope of the activation curve.
The characteristic features of divalent cation regulation in skeletal and cardiac muscle contraction are determined solely by the species of troponin C.
The Ca2+-sensitive ATPase activity of rabbit skeletal myofibrils disappeared completely after treatment with a solution containing CDTA, a strong divalent cation chelator, at a low ionic strength. A gel electrophoretic study revealed that all troponin C and about half of myosin light chain 2 were removed from the myofibrils by the CDTA treatment. The CDTA-treated myofibrils, when reconstituted with skeletal troponin C, showed almost exactly the same Ca2+- or Sr2+-sensitive ATPase activity as that of intact myofibrils. The CDTA-treated myofibrils reconstituted with porcine cardiac troponin C showed the same Ca2+- or Sr2+-sensitivity of the ATPase as that of porcine cardiac myofibrils; Sr2+-sensitivity relative to Ca2+-sensitivity was about ten times higher than, and the maximal slope of the activation curve was about half that of skeletal myofibrils. These findings indicate that these characteristic features of divalent cation regulation in the contraction of skeletal and cardiac muscles are determined solely by the species of troponin C. Bovine brain calmodulin hardly activated the ATPase activity of the CDTA-treated myofibrils even in the presence of Ca2+. Excess calmodulin, however, was found to give Ca2+- or Sr2+-sensitivity to the ATPase activity of the CDTA-treated myofibrils. Frog skeletal parvalbumins 1 and 2, even in excess, did not affect the ATPase activity of the CDTA-treated myofibrils.
Morimoto et al. (Sun,) reported a other. Substitution of Troponin C with Cardiac Troponin C, Calmodulin, and Parvalbumins vs. Intact myofibrils or skeletal troponin C reconstituted myofibrils was evaluated on Ca2+- and Sr2+-sensitive ATPase activity. Substitution of skeletal troponin C with cardiac troponin C in rabbit myofibrils increased relative Sr2+ sensitivity 10-fold and halved the maximal slope of the activation curve.